Plasterboard Calculator

How to Use the Plasterboard Calculator

1. Enter the room's length, width and ceiling height in metres.
2. Choose what you're boarding: walls, ceiling, or both.
3. Subtract openings. Add the number of doors and windows; the calculator uses typical sizes, which you can change.
4. Choose the board size and thickness. 2400 × 1200mm is the most common full sheet; 1800 × 900mm boards are easier to carry and fit into small spaces.
5. Choose how the walls are fixed: screwed to studs, or dot and dab on masonry.
6. Choose the finish: skim plaster, or taped and filled joints (ready to paint).
7. Set waste (10% is typical) and add prices (optional) for a cost estimate.

How Many Sheets of Plasterboard Do I Need?

Wall area = perimeter × ceiling height − doors and windows. Ceiling area = length × width.

Allow 10% waste for cutting around openings, sockets and corners, and 15% for rooms with lots of angles, sloping ceilings or boxing-in.

Which Plasterboard Thickness and Type?

Thickness

• 9.5mm: lightweight, commonly used on ceilings and walls with supports up to about 450mm apart.
• 12.5mm: the standard for most walls and ceilings, suitable for supports up to 600mm apart. If in doubt, use 12.5mm.
• 15mm: heavier, with better fire and sound resistance; often used for separating walls and fire-rated ceilings.

Types

• Standard wallboard: general use in dry rooms.
• Moisture-resistant (usually green): for bathrooms and kitchens. Use a tile backer board in direct water areas like showers.
• Fire-resistant (usually pink): where Building Regulations require fire protection, such as garage ceilings below bedrooms or loft conversion stairwells.
• Acoustic (usually blue): denser board for better sound insulation between rooms.
• Insulated plasterboard: board bonded to insulation, for dry lining cold external walls.

Edges: tapered edge boards are designed for taping and filling to a smooth, paint-ready finish. Square edge boards are usually skimmed with a coat of plaster.

Screws and Fixings

Plasterboard is screwed to timber or metal studs with drywall screws. A widely used specification is:

• Walls: screws at 300mm centres along each stud, with studs at no more than 600mm centres. Screws should be at least 10mm from bound edges and 13mm from cut edges.
• Ceilings: screws at closer centres, typically 230mm, because boards hang on their fixings.
• Screw length: about 25mm for a single layer of 12.5mm board on metal studs, longer for timber and double layers.

A 2400 × 1200 sheet on studs at 600mm centres needs about 27 screws (three studs × nine screws). On 400mm centres, it's about 36. Ceilings with joists at 400mm need roughly 42 per sheet.

Dot and dab fixes board straight onto masonry walls with blobs of plasterboard adhesive, instead of studs. You need adhesive rather than screws; a 25kg bag typically covers around 3–4 m², but check the bag.

Skimming, Tape and Filler

Skim coat. A finish plaster such as Multi-Finish is applied about 2mm thick over the boards. A 25kg bag covers roughly 10 m² at that thickness. The calculator rounds up and lets you change the coverage.

Jointing tape. Skimmed boards need scrim tape over every joint before plastering. Taped and filled (tapered edge) boards need paper or mesh tape and three coats of joint filler. A practical estimate is about 1.3m of tape per m² of board.

Joint filler. For taped and filled finishes, allow roughly 0.3kg of filler per m² of board.

Beads. Add angle bead for every external corner and stop bead where plaster meets another surface. Measure these lengths separately.

How Much Does It Cost to Board and Skim a Room?

Typical 2026 UK prices:

Boarding adds the cost of board, screws and labour on top of skimming. A small room usually takes about a day to skim; large or complex rooms take 2–3 days plus drying time. Leave fresh plaster to dry fully (often a week or more) and use a mist coat before painting.

Sloping Ceilings, Lofts and Awkward Rooms

Loft rooms and rooms under stairs are harder to measure because the walls and ceilings slope. Break the surfaces into simple shapes:

• Sloping ceilings: measure the length along the slope and multiply by the width. A slope 2.5m long on a 4m-wide room is 10 m².
• Gable ends and knee walls: treat triangles as base × height ÷ 2, and short knee walls as rectangles.
• Boxing-in pipes and steels: add the surface area of each box.

Enter the totals in the calculator's "Enter areas" mode and raise the waste allowance to about 15%, because angled cuts produce more offcuts. In loft conversions, ceilings and walls around the new stairway often need fire-rated board, and the roof slopes are usually lined with insulated plasterboard or board over insulation between the rafters, so check your Building Regulations drawings before ordering.

Tips for Boarding a Room

• Board the ceiling first, then the walls, so the wall boards support the ceiling edges.
• Stagger joints so they don't line up across the wall or ceiling.
• Keep the paper face out and the board's long edges running across ceiling joists.
• Lift wall boards about 10mm off the floor with a board lifter; the skirting will cover the gap.
• Don't over-drive screws through the paper face; set them just below the surface.
• Mark stud positions on the floor and ceiling before you cover them, so you can find them for shelves and skirting.

Insulating at the same time? The Insulation Calculator works out the rolls or boards you need. For a loft room, see the Loft Conversion Cost Calculator.

Frequently Asked Questions